1. Field of the Invention
The present invention relates to a SQUID (Superconducting QUantum Interference Device), and more particularly, to a SQUID sensor using an auxiliary sensor.
2. Description of the Related Art
Generally, SQUID is a device that uses the quantum interference effect and the Josephson effect of magnetic flux to respond to a variation of a weak magnetic field, and is used for a high sensitive magnetic sensor or biosensor. The SQUID has a measurement sensitivity of several fT/Hz1/2 when a low temperature superconductor is used, and several tens of fT/Hz1/2 when a high temperature superconductor is used.
However, the method using the SQUID has three drawbacks as follows.
1) One of them is a relationship between a Signal/Noise (S/N) ratio and an operation range. In other words, in principle, the SQUID can be used to design a driving circuit having a large S/N ratio such that the operation range can be wider, but since the sensitivity of the SQUID is degraded in a real circumstance due to influences of geomagnetic field generated in a general circumstance or magnetic field of a magnetic source, the S/N ratio is not increased as much as the operation range is increased. This is because the intensity of the geomagnetic field in the general circumstance is several tens of μT/Hz1/2, and the intensity of the geomagnetic field is only several mT/Hz1/2 according to a distance from a power supply or a distance from the magnetic source such as an electro-magnetic equipment, a car, etc.
The influences of various magnetic sources in the general circumstance can be excluded by a method of shielding the influences of the geomagnetic field or the magnetic source when the SQUID is cooled. However, a conventional shielding method using a multi-layered magnetic shield material has a drawback in that a high shielding cost is required and an ideal shielding is difficult due to a phenomenon that a magnetized amount resulted from a slow variation of the geomagnetic field or the magnetic field in a refrigerator driving unit is slowly varied, thereby causing a difficulty in an exact signal analysis in the SQUID.
2) Since the SQUID can be used at a temperature below 77K when the conventional superconductor is used, its use is possible only when refrigerant such as liquid nitrogen is used. That is, in order to use the SQUID for the purpose of a general usage or in a state of distant installation, it is essential to use the refrigerator that can continuously or frequently maintain a low temperature state without a periodical refrigerant supplement.
However, when the refrigerator is driven, the refrigerator itself generates a very large magnetic field. In case a general cryogenic refrigerator is used, since the intensity of the generated magnetic field reaches a level at which a property of the superconductor constituting the SQUID is deteriorated so that performance of the SQUID is remarkably deteriorated, the refrigerator should be used at a distance far from the SQUID. However, such a use causes the cooling efficiency to be abruptly dropped, so that the SQUID cannot be used in an appropriately state.
3) A conventional SQUID has a drawback in that since the SQUID has a different operation current in every device, the SQUID frequently shows a minute variation depending on a magnetic shield condition while a cooling process is performed using the refrigerator. That is, in appliance in the general circumstance using the refrigerator, even in the magnetic shield condition, since the magnetic field is rapidly varied at the time of initial cooling and resetting to degrade the sensitivity of the SQUID, the conventional SQUID has a drawback in that the self-noise of the SQUID is increased due to degradation of a material constituting the SQUID.
To solve the drawback of the self-noise increase, a flux-back setting method for allowing the magnetic flux applied to the SQUID to be constant is proposed to prevent a large magnetic field from being applied while the SQUID is normally operated. However, the SQUID using the flux-back setting method has a drawback in that an abnormal operation is caused by overload and at the time of resetting, a large variation in the magnetic field is caused in an instant and in most cases, magnetic flux is trapped in the SQUID at this moment, so that the SQUID is deviated from an optimal condition and noise is increased.
Accordingly, in order to overcome the above-described drawback of the noise increase, proposed is a method of using the auxiliary sensor operating in a relatively large magnetic field and at a room temperature to apply an offset magnetic field to the SQUID. Since the SQUID using the above method senses only a difference between a signal of the auxiliary sensor and its peripheral signal, the deterioration drawback in the sensitivity of the SQUID can have been solved.
However, the above-described conventional SQUID has a drawback in that since the auxiliary sensor has generally much more poor sensitivity than the SQUID, the noise of the auxiliary sensor is mixed with the offset magnetic field, so that a signal for measurement cannot be detected. Finally, this corresponds to the measurement using the sensitivity of the auxiliary sensor, so that the use of the SQUID is meaningless.
To solve the above drawbacks, the following methods are proposed.
One is a method in which an identical offset magnetic field is applied to a gradiometer including two SQUIDs disposed spatially apart from each other and the noise of the auxiliary sensor is offset by an output difference between two SQUIDs to measure a spatial variation of the magnetic field as the sensitivity of the SQUID. However, the conventional method has a drawback in that the gradiometer cannot obtain a magnitude variation of the magnetic field itself.
Another is a method in which an output of the auxiliary sensor is periodically digitalized to form the offset magnetic field and thereby prevent noise from being continuously applied. However, the method also has a drawback in that a separate logic circuit is needed for processing an instant offset magnetic field when the offset magnetic field is formed using a low noise current periodically digitalized, and the abrupt variation of the magnetic field does not provide an offset effect.